Many systems that use lasers to engage or image a remote target also require mechanisms for steering and directing the laser light over a wide operational field of view. In a typical such system, it is desirable to project a high power beam, typically 30-50 cm in diameter or more, over a wide field of view such as a 90-120 degree cone. Currently many high energy laser (HEL) systems use turrets as beam directors. A turret is usually large, heavy and disruptive to the aerodynamic properties of the platform it is mounted on. Turrets typically require a volume approximately three times the beam diameter on each side. In addition, a turret is usually mounted exterior to the carrying platform (e.g., an aircraft), and due to its wide volume of swing, it can substantially impair aerodynamic performance which effectively eliminates the option of supersonic HEL operation. One example is the Airborne Laser (ABL) laser turret mounted on the nose of a Boeing 747-400F aircraft. This turret is a large structure with a complex design. It measures 1.5 meters in diameter and weighs 12 to 15 thousand pounds.
In view of the beam directors that are currently in use, and the platforms they are used on, it is desirable to have a beam director which can be made smaller and has a significantly reduced weight.
Transmissive beam directors have the advantage of being able to be more compact than reflective beam directors. At present, the only optical material that has a low absorption and does not substantially thermally aberrate when used as part of an HEL system is low absorption fused silica. However, because of the low refractive index of fused silica, the prisms in prism-based beam directors end up being extremely thick and very heavy when the diameter of the beam director approaches 50 cm.